[test] Get multiline action input - parse as JSON

[emersonknapp]

Notes:
This is a test to try enabling generic solution for #526

• By using | pipe to start multiline string, we can have core.getInput return a long string.
• This returned string does not have line breaks - this means that YAML isn't really possible because it has syntactic newlines
• The reason to parse as JSON in the action is to validate the input will be readable by colcon way later - otherwise the feedback would be very slow
• After validation - we can write this file to $COLCON_HOME/defaults.yaml as per add colcon-mixin-name-build and colcon-mixin-name-test overrides #526. We could also write it anywhere and point to it via COLCON_DEFAULTS_FILE environment variable. Any JSON we expect here will be valid YAML as well.

[codecov]

Codecov Report

Merging #555 (5893153) into master (cf1700f) will decrease coverage by 2.22%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
- Coverage   42.42%   40.19%   -2.23%     
==========================================
  Files           2        2              
  Lines         198      204       +6     
  Branches       36       36              
==========================================
- Hits           84       82       -2     
- Misses        114      122       +8     

Impacted Files	Coverage Δ
src/action-ros-ci.ts	30.28% <0.00%> (-2.26%)	⬇️

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[christophebedard]

@emersonknapp this can be closed now that #570 has been merged